Conversion of Bioethanol to Olefins over ZrO2 Catalyst

Meixiang Gao,Jiaqi Xiao

State Key Laboratory of Biobased Material and Green Papermaking,Qilu University of Technology(Shandong Academy of Science),Ji'nan,Shandong Province,250353,China

Abstract: Ethylene and 1,3-butadiene are important organic chemical raw materials. Petroleum hydrocarbon cracking is the most widely used method for ethylene fabrication worldwide. In this study, the use of a ZrO2 catalyst,which can be easily synthesized, for the efficient conversion of bioethanol to ethylene and 1,3-butadiene was analyzed. X-ray diffraction, transmission electron microscopy, temperature-programmed desorption, and Fouriertransform infrared spectroscopy analyses were used to evaluate the surface properties of ZrO2.The selectivity of ZrO2 toward ethylene and 1,3-butadiene was as high as 98.2%. Catalyst selectivity toward dehydration or dehydrogenation products depends on the nature of the weak acid-base catalytic sites, whereas high-strength base and intermediate-strength acid catalytic sites selectively yield ethylene.

Keywords:bioethanol; ZrO2; bifunctional catalyst; ethylene; 1,3-butadiene

1 lntroduction

Bioethanol, which is a renewable, clean, and nonpolluting raw material, can be obtained from plant tubers, such as cassava, potato, and yam tubers; sugarcane;cereals; and some lignocellulosic plants[1]. Recently, the bioethanol production and purification methods have matured, and production has increased. Therefore, in addition to sugars, which can be easily fermented, non-fermented sugars or lignocellulose can also be used as raw materials for bioethanol fabrication[2].Moreover, many scholars worldwide are committed to exploring the feasibility of marine organisms, such as algae and seaweed, as raw materials[3]. Given the increasing demand for bioethanol, more effective technological methods based on different biomass sources should be developed.Therefore,the conversion of bioethanol to important downstream chemicals presents very broad development prospects[4]. Olefins, including ethylene and 1, 3-butadiene, present more important application potential than other downstream products[5].

Before petroleum was used for large-scale ethylene production, ethanol dehydration has been the main industrial method for ethylene production. Small and medium ethanol dehydration units are still used in Brazil, India, Peru, and other countries[6]. Owing to the increasing scarcity of petroleum resources, increasing prices, and increasing demand for environmental protection, new alternative or supplementary ethylene synthesis methods should be developed. Therefore,bioethanol dehydration to ethylene has attracted significant attention. Bioethanol degradation presents great economic benefits and strategic significance mainly because (1) biological resources are inexhaustible renewable resources, (2) significant progress has been recently made in biofermentation technology, and low-concentration ethanol raw materials are inexpensive and readily available, and (3)carbon dioxide emissions from this process are low[7].

In addition to ethylene, 1, 3-butadiene is another important organic chemical raw material, which is widely used in the synthetic rubber industry[8]. The recent developments in technology and military industry have led to an increasing demand for 1,3-butadiene. However, currently, the preferred industrial method for 1,3-butadiene fabrication is extraction from ethylene by-products, which mainly depends on nonrenewable petroleum resources as a single raw material source[9]. Since the 1970s, the increase in oil price,depletion of fossil fuels,and increase in pressure on the environment have caused widespread concern regarding the use of renewable resources[10].

Recently, researchers have designed several catalysts for the synthesis of ethylene[11-15]and 1,3-butadiene[16-20]from bioethanol. The latter reaction is more complex than the former, and no suitable catalyst for industrial production exists. This leads to a poor economy of the green synthesis method and restricts its development[21].Zhao et al[22]designed a ZnO-CeO2/SBA-15 catalyst for the synthesis of 1,3-butadiene from ethanol.A 1,3-butadiene selectivity of 45% was achieved when the performance of the catalyst was evaluated in a fixedbed reactor. Catalyst performance for ethanol dehydration to ethylene is an important factor in determining the economy of this method. Moreover,catalyst development is an important factor that restricts the development of 1,3-butadiene synthesis from bioethanol. To improve the conversion efficiency of bioethanol to important downstream olefins, in this study,a simple,inexpensive,and easy-to-fabricate ZrO2catalyst with dual acid-base functions was proposed.

2 Experimental

2.1 Synthesis of ZrO2 catalyst

After 10 g zirconium oxynitrate was weighed and added to 50 mL deionized water, a 25% ammonia solution was added to the mixture dropwise until it completely precipitated. The mother liquor was aged for 12 h at room temperature, filtered, washed, and dried for 5 h at 393 K in an oven. Subsequently, the dried product was placed in a box muffle furnace where it was calcined at 832 K for 6 h at a calcination heating rate of 5 K/min.

2.2 Characterization of ZrO2 catalyst

2.2.1 Transmission electron microscopy(TEM)

A Tecnai G20 F20 field-emission transmission electron microscopy (FE-TEM, FEI, The Netherlands)instrument was used to analyze the micromorphology of the catalyst samples.The sample preparation method was as follows.A few fully ground samples were added to ethanol for ultrasonic dispersion, and a drop of suspension was placed on a copper mesh to be tested after drying. The test conditions were as follows: an acceleration voltage of 200 kV, a point resolution of 0.248 nm,and a line resolution of 0.10 nm.

2.2.2 X-ray diffraction(XRD)

The crystalline structure of the catalyst samples was analyzed using a D/max-2500 XRD instrument(Rigaku,Japan) using Cu Kαradiation and a Ni filter at a tube voltage of 40 kV,tube current of 150 mA,and scanningrate of 0.05°/s,in the scanning range of 5°-90°.

2.2.3 Temperature programmed desorption(TPD)

Temperature programmed adsorption-desorption experiments were mainly used to estimate the acidity and alkalinity of the catalyst samples. The experiments were performed using an Auto II2920 automatic chemisorbent instrument (Micromeritics, US). Catalyst acidity was determined as follows. First, 100 mg catalyst sample was weighed and placed in a U-tube under a nitrogen atmosphere. Next, the temperature of the sample was increased to 500℃at a rate of 10℃/min and was maintained for 2 h,followed by decreasing the temperature to 70℃. Thereafter, an ammonia-argon mixture (ammonia content of 1% (V/V)) was added to the tube and was allowed to reach adsorptiondesorption equilibrium for 2 h. After nitrogen purging,the desorption response signal was recorded using a thermal conductivity detector in the temperature range of 100-500℃at a ramping rate of 10℃/min.

2.3 Evaluation of ZrO2 catalytic activity

Catalytic activity of ZrO2was evaluated in a fixed bed microreactor at atmospheric pressure,as described in the literatures[23-26].After the catalyst samples were pressed into tablets, 20-40 mesh granules were separated via grinding and sieving. Pressed catalyst samples (3 g)were placed in a reactor and pretreated with nitrogen.The nitrogen flow rate was 40 mL/min,and the reaction temperature was increased to 623 K at a ramping rate of 5 K/min.The temperature was maintained for 2 h.

Next, the reaction material was injected into the vaporization chamber of the reactor using a metering pump, and the vaporization temperature was 398 K.After the reaction reached steady-state under the specified evaluation conditions, the reaction products were flown through a heat preservation pipeline into a 7890A gas chromatograph(Agilent,US).An HP-PLOTQ column(0.535 mm i.d.,3 μm thickness,30 m length)was used for on-line analysis.

3 Results and discussion

3.1 Physical and chemical properties of ZrO2 catalyst

To evaluate the micromorphology of the ZrO2catalyst,the prepared catalyst samples were analyzed using the aforementioned FE-TEM instrument, and the results are presented in Fig. 1. The ZrO2catalyst samples consisted of relatively uniform small particles with a size in the range of 10-20 nm. The relatively uniform small particles provided reactive sites for bioethanol conversion to olefins, which created an advantage in the microstructure for conversion of bioethanol to bulk chemicals.

Fig.1 FE-TEM image of ZrO2 catalysts

The specific surface area and pore structure of the ZrO2catalysts were analyzed, and the results are presented in Fig. 2. The BET specific surface area of the ZrO2catalysts was relatively small (only 53 m2/g).The reverse shape of the N2adsorption-desorption curve was latter, which indicated that the pores in the ZrO2catalyst were mainly particle accumulation pores.

Fig.2 N2 adsorption-desorption curves of ZrO2 catalyst

To analyze the crystal structure of the ZrO2catalyst samples, the catalyst samples were subjected to XRD analysis, and the results are presented in Fig. 3. The ZrO2crystals prepared via coprecipitation consisted of a combination of monoclinic, tetragonal, and cubic particles.The presence of different crystal phases in the ZrO2catalyst sample reflected the ability of the ZrO2catalyst to convert bioethanol to different olefins.

Fig.3 XRD pattern of ZrO2 catalyst

Fig.4 Acidic properties of ZrO2 catalyst

ZrO2is a typical acid-base bifunctional catalyst[27-28].First, the acidity of ZrO2was investigated using NH3-TPD and pyridine (Py)-Fourier-transform infrared (FTIR) analyses, and the results are presented in Fig. 4.Two NH3desorption peaks at 120 and 310℃were observed during temperature-programmed heating. The results indicated that two types of acidic sites, namely weak and medium acid, were present on the catalyst surface.The peaks at 1540 and 1450 cm-1in the Py-FTIR spectrum gradually disappeared with increasing temperature, indicating the presence of Brønsted and Lewis acid sites on the catalyst surface. These results were also directly related to the high selectivity of ethylene in subsequent activity tests, as described in detail in Section 3.2.

Fig.5 Basic properties of ZrO2 catalyst

The surface alkalinity of the ZrO2catalyst was investigated using CO2-TPD and CO2-FT-IR (Fig. 5).Four desorption peaks at 100, 260, 305, and 480℃,were observed in the CO2-TPD spectrum, which indicated the presence of four types of alkaline sites on the catalyst surface,with alkalinity ranging from low to high. A single CO2-FT-IR peak appeared at 1650 cm-1,and gradually disappeared with increasing temperature.The peak indicated the presence of a weak alkaline site on the catalyst surface, whereas the other types of alkaline sites could not be observed in the CO2-FT-IR spectrum. This was attributed to the intensities of the CO2-FT-IR peaks being lower than the detection range of this method. In addition, the presence of strong alkaline sites on the ZrO2catalyst surface was ascribed to the high selectivity of the catalyst toward described,which was analyzed in detail in Section 3.2.

Table 1 Product distribution of bioethanol reaction over the ZrO2 catalyst

3.2 Bioethanol reaction on the ZrO2 catalyst surface

The reaction of bioethanol on the surface of the ZrO2catalyst was analyzed at a reaction temperature of 350℃and a feed rate of 1.8 h-1. The selectivity of the catalyst toward ethylene and 1,3-butadiene was 98.2%,and product distribution was simple. There is neither ethylacetate, propylene, butene or butanol which is usually produced in this reaction system. The main products were ethylene and 1,3-butadiene; however,small amounts of ether (carbon selectivity of 1.36 C mol%)and a high carbon product(carbon selectivity of 0.41 C mol%) were generated (Table 1). More energy is saved for product separation.

The aforementioned medium-strength acid sites and strong alkaline sites on the surface of the catalyst were suitable active sites for ethanol dehydration to ethylene.Moreover, the presence of weak acid-base catalytic sites favored the production of 1, 3-butadiene from bioethanol, which has been studied in our previous work[24].

In conclusion, the ZrO2catalyst presented high catalytic selectivity for ethylene and 1,3-butadiene. If the specific surface area of the ZrO2catalyst were increased, a higher conversion could be achieved by loading or using other preparation methods. Currently,in industry, 1,3-butadiene is fabricated via ethylene byproduct fractionation. The ZrO2catalyst can effectively convert bioethanol into ethylene and 1, 3-butadiene,and, therefore, it is expected to be an attractive catalyst for the industrial conversion of bioethanol to dienes.Nevertheless, the ZrO2catalyst presents great research significance.

4 Conclusions

The green conversion of bioethanol to important downstream chemicals was analyzed, and the catalysts for the synthesis of ethylene and 1,3-butadiene were fabricated, and the ZrO2catalyst with a good economic performance was selected for this study. The results revealed that the ZrO2catalyst presented acid-base bifunctional properties,medium-strength acid sites,and strong alkaline sites, which were suitable active sites for ethanol conversion to ethylene. Moreover the weak acid-base sites on the ZrO2catalyst surface were suitable active sites for ethanol conversion to 1, 3-butadiene. Because the selectivity of the ZrO2catalyst for ethylene and 1,3-butadiene was very high (98.2%),we expected that this study will provide research direction for green bioethanol conversion to important downstream chemicals.

Acknowledgments

This study was supported by the Foundation of the State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology/Shandong Academy of Sciences (grant no. ZZ20190319) and the Foundation of the Natural Science Foundation of Shandong Province(grant no.ZR2020QB190).